A typical conventional image sensor used for facsimile machines or the like is a contact type image sensor, which provides electric signals representing a 1:1 image of an original. A TFT (thin film transistor) driven image sensor, which falls under the category of the contact type, has been proposed in the art. In a TFT driven image sensor, the image of the original is segmented into a large number of picture elements (pixels) and falls upon photosensors, each of which corresponds to a particular pixel, to cause electric charges to be induced therein. In response to the energization of switching elements, each of which is composed of a TFT, the induced electric charges in a given block of photosensors are transferred to and temporarily held in associated line capacitances pending output. The stored electric charges therein are sequentially output in the form of electric signals by the action of a driver integrated circuit (IC). A matrix configuration (i.e. a two dimensional addressing scheme) is used to drive the TFTs, which allows a single driver IC to deal with a plurality of blocks of photosensors. For this reason, this type of TFT driven image sensor is requiring fewer driver IC chips for driving the image sensor.
Referring to FIG. 8 which illustrates an equivalent circuit diagram of a known TFT driven image sensor of the type mentioned above, it will be observed that the TFT driven image sensor includes a photosensor array 81 having a row of photosensors Pi,j for i=1 . . . k, j=1 . . . n, the length of which is substantially the width of an original. A charge transfer section 82 is also provided, which consists of thin film transistors Ti,j for i=1 . . . k, j=1 . . . n, one for each photosensor. The TFT driven image sensor further includes a matrix configuration multilayered wiring section 83.
The photosensor array 81 is divided into k groups of photosensors, i.e. into k blocks, and each of the blocks consists of n photosensors. Each photosensor Pi,j for i=1 . . . k, j=1 . . . n may be regarded as equivalent to photodiode and its parasitic capacitance. Each photosensor Pi,j for i=1 . . . k, j=1 . . . n is connected to the drain electrode of the corresponding TFT Ti,j for i=1 . . . k, j=1 . . . n. The source electrodes of the TFTs in each block are connected to n common signal lines 84 by the matrix configuration multilayered wiring section 83. The source electrode of the first TFT in each block is connected to the first common signal line, the source electrode of the second TFT in each block is connected to the second common signal line, and so forth. The common signal lines 84, in turn, are connected to a driver IC 85. The gate electrodes of the TFTs are connected to a gate pulse generating circuit 86 so that each block can be sequentially operated.
The photoelectric charges generated in each photosensor Pi,j for i=1 . . . k, j=1 . . . n are temporarily held by the parasitic capacitance of the photosensor and the drain/gate overlap capacitance of the corresponding TFT Ti,j for i=1 . . . k, j=1 . . . n. The photoelectric charges held in a given block are transferred to the multilayered wiring section 83 and held in the line capacitances indicated in the figure by CL in response to the energization of the TFTs in the block which function as switches for transferring the electric charges. All the TFTs in a given block are rendered conductive at a time by simultaneously applying a gate pulse to all gates of the TFTs in the given block, which allows each block to be activated sequentially.
Specifically, TFTs T1,1 to T1,n in the first block are turned on in response to a gate pulse .phi.G1 transmitted on gate signal line G1 from the gate pulse generating circuit 86. The electric charges induced in the photosensors in the first block are transferred to the line capacitances CL and held therein. The electric charges in the line capacitances CL change the potentials of the associated common signal lines 84. Resultant electric voltages are detected in serial order and output onto an output line 87 by sequential turning-on of analog switches SWj for j=1 . . . n in the driver IC 85.
Similarly, TFTs T2,1 to T2,n through Tk,1 to Tk,n in the second block through the kth block are sequentially turned on in response to respective gate pulses .phi.G2 to .phi.Gk. The electric charges induced in the photosensors are transferred and read out block by block, thereby to obtain image signals reflecting a whole line with respect to the fast scan direction of the scanned document. The original to be scanned is then moved by a suitable automatic document feeder (not shown) such as a roller. These operations as mentioned above are repeated, to obtain image signals representing the entire document scanned (Japanese Patent Laid Open No. Sho. 63-9358).
One variation on the aforementioned TFT driven image sensor is a color image reading apparatus using the same basic mechanism, whose structural detail plan view is shown in FIG. 7. It will be seen that the color image reading apparatus has three rows of photosensor arrays 81a, 81b and 81c aligned in the slow scan direction. There is one switching element T for each photosensor of the arrays, and three switching elements T in the column for each pixel have their source electrodes S connected to a signal output line 90. The signal output lines 90 are laid along the slow scan direction between each of the photosensors, so that the photosensor arrays 81a, 81b and 81c can be positioned in parallel to the matrix wiring section 83 shown in FIG. 8. Each row of the photosensor arrays 81a, 81b and 81c is further provided with a linear color transmission filter (not shown) which is transmissive to selected colors, i.e. light of certain wavelengths. Three color transmission filters are employed to distinguish for example the primary colors red, green, and blue, thereby to provide image signals for each color.
This structure of the color image reading apparatus known in the art, however, suffers from various shortcomings and disadvantages. One problem of the prior art is that the color image reading apparatus involves a relatively large capacity of a line memory means incorporated therein. In order to regenerate image signals from color-separated image signals, the color-separated image signals supplied from each of the common signal lines must be temporarily held in the line memory means until the color-separated image signals will complete a set, i.e. one pixel. This problem is caused by the presence of relatively wide spatial gaps in the slow scan direction, which are occupied by the switching elements T intervening between each of the photosensor arrays 81. These spatial gaps consequently cause time delays, as each line of an original is scanned by the photosensor arrays 81 in turn.
Another problem of the prior art is variation in the amount of light incident upon individual photosensors of the arrays 81. In operation, light reflected by a scanned original is concentrated on the photosensor arrays 81 by a lens system (not shown), such as a rod lens array of the type produced by Nippon Sheet Glass Co. Ltd. (Japan) under the name of Selfoc Lens, which is fixed above the color image reading apparatus. As explained above, the three rows of the photosensor arrays are spaced apart in the slow scan direction, resulting in nonuniform light intensities. Even if the intensity of the reflected light is uniform, the amount of light incident upon the photosensor arrays 81 depends on the positions of the arrays.